Convex zero grind and sandpaper

J

JeffreyFogleboch

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May 13, 2009
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193
Hey guys I've been trying to get a nice sharp edge on my convex zero grind Japanese style blades for a while and I just can't seem to get them sharp. They are sharp off of the grinder but when I hands and they are always full or only fairly sharp. I keep an eye on where I am sanding and have tried a hard backing and a leather backing. How do you guys get them sharp? Is there a trick to not roll over the edge? I know a lot of bowie makers do a variation of this geometry so I would greatly appreciate any advice! Thanks!

-Jeff


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Jeff - what kind of knives are you talking about - kitchen knives or outdoor knives? Some photos would help. One waz or another - I would finish sharpening on water stones - especially with kitchen knives.
 
I'm talking about like tanto and wakizashi made more or less traditionally.
3bdecd44f6c8c72e696745b69e84a49c.jpg
here is a picture of the current blade I am polishing/sharpening. I know it can be done with stones but I don't have the training or patience for that. Thanks guys!


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That blade looks really nice. I would definitely be sharpening that on stones (after the surface is finished). I would guess that grinder is too 'coarse' of a tool for final sharpening. But yes, it would be some work.

Did you consider to have it sharpened by someone experieced that offers such services for japanese kitchen knives?
 
nice looking blade ! my first guess would be that you are going too far over the edge and rounding it off instead of coming to zero. i think most guys do convex on a belt ? what is your technique.
 
i just imagined this in my head. i have never tried it, but it should make a consistent curve and might be worth a try. i would use a 8-10" diameter or larger curve ? i used a 4 " pvc just to illustrate the idea. if you glue a thin strip of sandpaper to the pvc, lay and hold the blade flat on the pvc with the sandpaper barely going over the edge of the blade, and pull toward the tang making sure it stays flat , it should make a consistent curve without going over. again, just an idea to try if nothing else works out.
 

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Right now I'm doing what most people are which is to use a backed piece of sandpaper just as if you were handrubbing the blade and slowly working my why down to the edge so the whole bevel gets polished and hopefully sharper at the same time. That's a good idea with the sandpaper but idk if I could get the curve exactly how I want it


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Why does the yokote look so washed out in that picture? And I note there's no hole in the tang yet- was this blade newly forged by a custom smith? I have to believe it would be better to practice on cheaper production blades first, until you get your technique down.

That said, my guess would also have to be that you're rounding the very edge apex a little too much. It's easy to do. Plus, as you progress to finer grits it should start to seem sharper since a coarse edge won't grab arm hair as easily.
 
The yokote is washed out bc I haven't quite figured how to keep it nice and crisp. I haven't drilled the hole yet. This blade is forged by me


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As best as I can tell, the tradition japans way of getting that "zero convex" blade profile is kind of the same way that Bill Moran used to do it, but on water stones. You "cut" a series f flat facets and then go back and blend them. On the coupelfo ktiuchn knives hat i have done with complex grind like the S grind, I found that 1/2 inch to 1 inch wide EDM stones work very well for that blending process, followed up by fine sandpaper rung lengthwise.
 
On my convex knives I finish them by slack grinding with an A30 trizact, then switch to a felt belt with white compound on it, then finish by stropping on a leather strop with green compound. That seems to work pretty well. Here's me trying to demonstrate sharpness of the first blade I did that way.

[video]https://youtu.be/vUv16IqZSeQ[/video]
 
I use this type of edge a lot and have had good success with it! There's a facebook video of me demonstrating slicing w/ this same grind, you can find me under Roger Walco.
I'm glad to chat if I can be of help. Hint: I do my finish sharpening with very fine water stones after the sanding.

For a Japanese type blade, you do not want to be using buffing wheels at all. You want your grain pattern to stay crisp. A little compound on a strop is fine for final edge, though.

Roger
 
I sharpen convex edges on flat diamond stones and diamond/ceramic rods. Also use a flat sharpener with rounded sides I make, made from hardened D2 steel. Convex edges need to be sharpened at a higher angle hitting only the edge when using these type of sharpeners.
Scott
 
You don't have to try to get a convex edge to get a convex edge. The amount of convexity you're shooting for is extremely little.

Most guys who sharpen on stones figure there's enough human error to account for all the convexity they want.

Many guys who used sandpaper on a mouse pad now use compound on leather because the former produced too much convexity, and the "give" in leather produces enough convexity.

The guys using belt grinders hardly use any pressure against the belt, so it deforms very slightly. Again, that seems to be the right amount.

Trying to get more convexity gives more obtuse edge geometry, which is another way of describing "rounding the edge".
 
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